Metal purification



Patented July 26, 1938 UNITED STATES METAL PURIFICATION Harvey N. Gilbert and Norval D."-Clare, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing.

Application November 4, 1936, Se-,

rlal No. 109,128

11 Claims.

This invention relates to the purification of alkali metals and more particularly to a method for removing mercury from mixtures of alkali metal and mercury.

Amalgams obtained by using mercury as a cathode for electrolysis of alkali metal salt solutions. usually contain notmore than about 0.5% of alkali metal, since at higher concentrations the amalgam is not easily maintained in liquid state at temperatures at which it is convenient to operate the electrolysis cell. Various methods have been proposed for recovering alkali metal from these dilute amalgams, some of which comprise heating to distill oil the mercury and others of which comprise utilizing the amalgam as an anode in a second cell with a suitable electrolyte, for example, fused alkali metal hydroxide. In such methods it is diflicult to obtain alkali metal which does not contain appreciable amounts of mercury. In order o tions of alkali metal salts, using a mercury cath--,

ode. A further object is to provide an improved method for recovering substantially pure alkali metal from alkali metal amalgams. object is to purify alkali metal which is contaminated with small amounts of mercury.

Other objects will be apparent fromthe follow-.

ing description of our invention.

The above objects are attained by our invention as hereinafter described by subjecting a mixture of mercury and alkali metal, while maintained substantially in the liquid state at a temperature above the' melting point of the alkali metal present, to the action of a substance which will react with the mercury to form a solid, mercury-containing material but which is substantially chemically inert towards alkali metal and separating said solid mercury-containing material'from the liquid melt. As reagent for this process, we prefer to use an alkaline earth metal. We have discovered that if a mixture of mercury and alkali metal is contacted with an alkaline earth metal at a temperature above the melting point of the alkali metal at which the mixture is liquid, the mercury in the amalgam combines'with the alkaline earth metal to form a Another solid substance which may readily be removed from the melt by mechanical means such as filtering, centrifuging or settling.

In the present specification and in the appended claims we use the term alkaline earth metal to include magnesium as well as calcium, strontium and barium, since we have found that magnesium is suitable forour method as well as the other three'metals named. However, we ordinarily prefer to use calcium in our separation method, because it is cheaper than strontium or barium and usually smaller amounts of it are required than of magnesium.

In practicing our invention, we may mix an alkaline earth metal, preferably in a finely divided form, with a molten alkali metal amalgam and permit the mixture to stand until the equilibrium conditions are attained. The mixture then is filtered or otherwise suitably treated to remove liquid alkali metal from the solid substance formed. The solid material thus removed, which contains chiefly alkaline earth metal, mercury, and usually some adherent alkali metal,

may be heated in a retort to recover the mercury as mercury vapor which may be condensed, leaving a. residue of alkaline earth metal, together with a small amount of alkali metal and possibly some mercury. This alkaline earth'metal residue then may be used again without further treatment to remove mercury from another lot of Y alkali metal amalgam.

In practicing this separation, we prefer to have present in the mixture undergoing treatment ,sufiicient alkali metal'so that the mixture may at all times be maintained substantially in the liquid state at a temperature between the melt- Cir ing point of the alkali metal andfabout 300 C. I

Hence, if the alkali metal amalgam to be treated does not contain sufilcient alkali metal to attain this condition, we prefer to add alkali metal to the melt until the mixture is sufiiciently liquid to be readily filtered; For greatest convenience in operatidn, we prefer to maintain the alkali metal content at not less than about 75% of the total weight of the mixture. The additional alkali metal required may be added at any time during the treatment, prior to separation of the solid, mercury-containing material. We

' prefer to add the amalgam to the desired quantity ofmolten alkali metal and then to mix in the alkaline earth metal.

In order to recover an alkali metal from its amalgam by our novel method, the weight of the alkaline earth metal contacted with the molten amalgam should be greater than the weight of the mercury in the amalgam. We have. found, for example, that when calcium is utilized to remove mercury from a sodium amalgam the calcium will combine with about one-fifth to onefourth of its weight of mercury whenthe operation is carried out at a temperature of about 200-250 C. Therefore; if it is desired to remove substantially all of the mercury in a single treatment at 200-250 C., the amount of calcium taken should be equal to at least 3 to 5 times the weight of the mercury present. Larger amounts of calcium may be used if desired; but ordinarily amounts greater than to times the weight of the mercury are not necessary. Similar amounts of other alkaline earth metals will be required; the minimum amount will depend on the nature of the metal and the amalgam. In general, we prefer to use an amount of alkaline earth metal equal to not less than about three times the weight of the mercury it is desired to remove. Smaller amounts may be used if desired; but generally, such smaller amounts will not be effective in removing all of the mercury by a single treatment.

In contacting the alkaliipetal amalgam with the separation reagent, the time required for equilibrium conditions to be attained will vary,

depending upon the degree of subdivision of the reagent, the nature of the subdivided material and the temperature at which the mixture is maintained. For example, we have found that, although the reaction proceeds at temperatures at or close to the melting point of the alkali metal present, the equilibrium maybe reached much more quickly if a higher temperature is used, preferably about 200 to 300 C. Even higher temperatures may be used if desired; as will appear hereinafter, in some cases we may use temperatures as high as 600 to 800 C. However, we prefer to cool the mixture to a temperature below about 300 C. but above the melting point of the alkali metal before separating the solid reaction product.

We have further discovered that the method by which the alkaline earth metal or other reagent is subdivided is an important factor in the time required for reaching equilibrium. I That is, we have found that if the alkaline earth metal is subdivided by precipitating it from its solution in molten alkali metal, the time of treatment and the amount of alkaline earth metal required for' the treatment both are considerably reduced. We therefore prefer to carry out our process by treating the amalgam with finely divided alkaline earth metal which has been prepared by precipitating it from its solution in a molten alkali metal. Such precipitates may be readily obtained by add- I ing an alkaline earth metal to a bath of molten alkali metal, preferably under an atmosphere of inert gas such as nitrogen, at such temperature as will cause the dissolution of considerable amounts of alkaline earth metal, e. g. at 500 to 800 C., and then cooling the melt to a temperature just above the melting point of the alkaline earth metal. The resulting mixture of precipitated alkaline earth metal ,and molten alkali metal then may be used per se as the treating agent or, if desired, the alkali metal may be filtered by known means to leave a sludgy residue consisting of the precipitated alkaline earth metal, wet with the alkali metal and this residue or .gludge may be used as the treating. agent.

In general, we have found that the reaction between the mercury and the reagent used to remove it is greatly facilitated by finely subdividing the reagent substantially out of contact of air and moisture or other gas or liquid reactive therewith. Preferably, this may be done by carryingout the subdivision under the surface of a bath of molten alkali metal, which may or may not contain mercury, such subdivision may be accomplished by the precipitation method described in the preceding paragraph (when the reagent is soluble in the alkali metal) or other means of subdivision may be used, e. g. mechanical means such as cutting or abrading.

By way of example of one method of practicing our invention, we shall describe the treatment with calcium of metallic sodium containing around 1% of mercury to produce substantially pure sodium. First, a calcium sludge is produced by dissolving calcium in a quantity of pure sodium under a nitrogen atmosphere at a temperature of 500 to 700 C., cooling the melt to about 110 C. and filtering through a fine mesh steel wire cloth to recover the precipitated calcium sludge. The impure sodium to be treated then is heated to about 250 C. and a quantity of the calcium sludge, containing from 3 to 5 parts by weight of calcium for each-part of mercury in the impure sodium, is added with thorough stirring. This operation preferably is carried out in an atmosphere of nitrogen or other suitable inert atmosphere in order to prevent contamination by oxide formation. After one to ten minutes, the mixture may be filtered, either at the treating temperature or any temperature between that and the melting point of the sodium. The

filtrate will be found to be substantially free from mercury. The resulting filter residue may be distilled to drive off mercury therefrom, leaving a,

'mixture of calcium and sodium substantially identical to the above described calcium sludge. This residue then may be used for a further treatment.

The following examples will further illustrate our invention:

Example 1 impure metal is cooled to a few degrees above the melting point of sodium to cause dissolved calcium to precipitate and then filtered. The filter residue is a 'sludgy mixture of calcium crystals, mixed with molten sodium and small amounts of oxide and salt.

Portions of impure sodium containing 1.42% of mercury were melted and heated to 200 C. To each was added a measured quantity of the calcium sludge which was thoroughly stirred into the melt. After standing at about -200 C.v for a measured length of time, each melt then was fil tered at 110 to 200 C., using glass wool as fllter medium. The resulting filtrates were analyzed for mercury. The following results were obtained:

A barium sludge was prepared by heating 3.8 grams of pure barium metal with molten sodium,

850 C. for 1 hour and then cooled to about 200' C. to precipitate the dissolved barium. Then 50 grams of sodium containing 1.2% of mercury was added to the sludge and the mixture was agitated for 15 minutes at 250 C. The melt then was cooled and filtered through glass wool at 100 110 C. The filtrate was found to contain 0.079% by weight of mercury.

Ezample 3 A magnesium sludge was prepared by stirring 59 grams of magnesium with 75 grams of molten sodium at about 700 C. under an atmosphere of nitrogen for 30 minutes, to dissolve magnesium.

The melt was then cooled to 200 C. and mixed with 75 grams of molten sodium .containing 1.2% by weight of mercury. The melt then was maintained at 200 C.'with occasional agitation for 15 minutes. The melt then was filtered at about 110 C. The filtrate contained 0.048% by weight 01' mercury.

Our herein described invention is especially useful for purifying alkali metal contaminated with small amounts of mercury. e. g. around 5% or less, to produce substantially pure metal. It thus may be used in conjunction with other methods for removing mercury from alkali metal amalgams, e. g. by using it to remove the last traces of the mercury. However. our invention is not restricted to the removal of small amounts of mercury from alkali metal. but also may be used to remove mercury from alkali metal amalgams containing large amounts of mercury. I

It is understood that our invention is not restricted to the specific methods described herein by way of example, since various modifications will be apparent. In general, our invention comprises a method wherein a mixture of alkali metal and mercury is reacted-with a material capable of forming a solid mercury-containing substance which may be filtered oil or otherwise mechanically separated from the molten alkali metal.

Moreover, it is obvious that the process is not restricted, to the removal of mercury from a single alkali metal but also may be used to separate mercury from various sodium alloys, e. g., a mixture or alloy of two or more alkali metals, such as sodium-potassium alloy.

We claim:

1. A process for removing mercury from a mixture of mercury and an alkali metal comprising contacting said mixture with a metal which will react with mercury to. form an amalgam which is substantially immiscible with said alkali metal and separating said amalgam from said alkali metal.

'2. A process for removing mercury from an alkali-metal-mercury mixture which is composedpreponderantly of said alkali metal comprising contacting said mixture at a temperature above its melting point with a metal which reacts with mercury to form an amalgam substantially insoluble in the molten alkali metal and removing the insoluble amalgam from said alkali metal.

3. A process for removing mercury from a mixture of mercury and an alkali metal comprising contacting said mixture with an alkaline earth metal at a temperature above the melting point of said mixture and thereafter separating solid, mercury-containing material from the melt.

4. A process for removing mercury from a mixture of mercury and an alkali metal comprising contacting said mixture with calcium at a temperature above the melting point of said mixture and thereafter filtering the melt to remove insoluble material containing mercury and alkaline earth metal.

5. A process for removing mercury from a mixture of mercury andan alkali metal comprising mixing therewith a finely divided alkaline earth metal at a temperature above the melting point of said mixture and thereafter separating solid, mercury-containing material from the melt.

6. A process for removing mercury from a mixture of mercury and sodium comprising mixing therewith finely divided calcium at a temperature above about 200 C. and thereafter filtering the melt to remove solid material containing mercury and calcium.

7. A process for removing mercury from a ing the melt to remove solid material containing mercury and calcium.

8. A process comprising cooling a solution of an alkaline earth metal in molten alkali metal to cause precipitation of said alkaline earth metal, contacting the precipitated metal with a molten mixture of mercury and alkali metal and thereafter removing solid, mercury-containing material from the melt.

9. A process comprising cooling a solution of an alkaline earth metal in molten alkali metal to cause precipitation of said alkaline earth metal, filtering oi! the precipitated metal from the melt. adding to a molten mixture of mercury and alkali metal at least 3 parts by weight of said precipitated metal for each part by weight of mercury in said molten mixture and thereafter filtering of solid, mercury-containing material from said molten mixture.

10. A process comprising cooling a solution of an alkaline earth calcium in molten sodium to cause precipitation of calcium, filtering off the precipitated calcium from the melt, adding to a molten mixture of mercury and sodium about 3 to 5 parts by weight .of said precipitated calcium for each part by weight of mercury in said molten mixture at a temperature 200 to 300 C. and thereafter filtering oif solid, mercury-containing material from said molten mixture. I v

11. A process comprising cooling a solution of an alkaline earth metal in molten sodium to cause precipitation of said. alkaline earth metal, contacting the precipitated metal with a molten mixture of mercury and sodium at a temperature of not less than about 200 C. and thereafter removing solid, .mercury-containing material from said molten mixture.

- HARVEY N. GILBERT.

NORVAL' p. CLARE. 

